1. Field
The following description relates to a conductive paste containing silver nanoparticles, and in particular, to a conductive paste containing silver nanoparticles and silver-decorated carbon nanotubes.
2. Description of Related Art
Information communication devices, such as liquid crystal displays, are becoming more miniaturized with higher performance. Attempts have been steadily made to incorporate these devices on flexible material supports. The circuit wire of the devices is generally formed by forming a film by vapor deposition such as chemical vapor deposition (CVD), sputtering and so on, and etching out an unnecessary portion by photolithography and so on.
However, the conventional method for forming a circuit wire has disadvantages caused by repetition of film formation and etching, such as a low usage efficiency of raw materials, generation of a large amount of waste, a long manufacturing time, and a considerable installation cost. Further, the conventional method encounters many difficulties in forming a fine circuit wire required for miniaturization of the aforementioned devices.
To solve the above-referenced issues, recently, the related industries have been focusing on ink-jet printing, gravure printing, and screen printing techniques that allow a low loss of raw materials, non-use of hazardous components, such as lead or the like, and a simple process for forming a circuit wire. To form a circuit wire by these techniques, development of a high-performance conductive paste or ink is needed.
A conductive ink suitable for forming a circuit wire should have a high conductivity represented as a specific resistivity of 1×10−5Ω·cm or less. Conventionally, a conductive paste or ink was suggested to include a large amount of silver particles occupying 50 to 80% of ink weight so as to accomplish a continuous metallization. However, to form a continuous conductive network with solely silver (d=10.5 g/cm3), 75 weight % or more of silver is needed. The use of a large amount of silver produces very disadvantageous results in aspects of cost and storage stability.
Further, when printing the conductive paste on a flexible circuit board, a sufficiently low sintering temperature is important, because plastics or the like have a low glass transition temperature (Tg). The smaller metal particles have higher surface energy, and, accordingly, the sintering temperature tends to be even lower than an intrinsic melting point of a metal. This tendency is illustrated in FIG. 1. However, if a high content of silver particles and a reduction in particle size to a nanometer level is applied for a high conductivity and a low sintering temperature, agglomeration of silver particles is accelerated. Thus, to obtain storage stability of a conductive paste or ink, it is inevitable to use an additive such as a dispersion, a stabilizer, and so on. However, the additive increases the sintering temperature again, which was low once due to particle size reduction of silver.